The present disclosure relates to a lithium ion secondary battery and, more particularly, to a positive electrode which can be applied to a lithium ion secondary battery.
In recent years, in association with the spread of portable information electronic apparatuses such as the cellular phone, the video camera, notebook-sized personal computers, and the like, the realization of advanced performance, miniaturization, and light-weight of the apparatus has rapidly been progressing. An expendable primary battery or a secondary battery which can be repetitively used, is used in each of power sources which are used in those apparatuses. Among them, a demand for a lithium ion secondary battery is increasing.
In recent years, in order to realize the higher advanced performance of the portable information electronic apparatuses, particularly, (1) realization of a high-energy density of the lithium ion secondary battery and (2) improvement of cycle characteristics are demanded. Herein, (1) the realization of the high-energy density of the lithium ion secondary battery and (2) the improvement of the cycle characteristics are explained.
(1) The Realization of a High-Energy Density of the Lithium Ion Secondary Battery.
Use of a positive electrode having a large discharge capacitance per unit volume is one effective method for realizing high-energy density. To realize such a positive electrode, it has been known that (a) selection of an active material; and (b) an increase in charge upper limit voltage are important. In recent years, much attention has been paid to studying high energy by increasing a charge in upper limit voltage.
As far as cathode active materials of the lithium ion secondary battery, besides LiCoO2 and the like, LiNiO2, LiMn2O4, and the like are known. According to LiNiO2, although its capacitance is equal to about 190 mAhg−1 and is relatively large, it is necessary to reduce the discharge cut-off voltage in order to obtain the capacitance. However, since an average discharge voltage is low, it is unsuitable for an application of the notebook-sized personal computer and the like in which a high electric power is necessary. According to LiMn2O4, since its capacitance is small, it is unsuitable for the purpose of realizing the high-energy density of the lithium ion secondary battery.
For the above reasons, a transition metal oxide containing lithium such as LiCoO2 or the like in which the average discharge voltage is high is desirable as a lithium ion secondary battery of a high-charge voltage for use in application of the notebook-sized personal computer. In the lithium ion secondary battery in which LiCoO2 is used as a cathode active material and a carbon material is used as an anode active material, a charge final voltage lies within a range from 4.1 to 4.2 V. In such a charge condition, the capacitance of the positive electrode of only about 50 to 60% of a theoretical capacitance is used. Therefore, if the charge voltage can be raised, the capacitance of the positive electrode of 70% or more of the theoretical capacitance can be used. The high capacitance and the higher-energy density of the lithium ion secondary battery can be realized.                For example, as disclosed pamphlet of International Publication WO03/019713, a fact that the high-energy density appears by setting the voltage upon charging to 4.30 V or higher is known.        
(2) The Improvement of the Cycle Characteristics
In the lithium ion secondary battery, an assembly in which an aluminum foil serving as a collector has been coated with a positive electrode mixture consisting of a cathode active material such as a transition metal composite oxide containing lithium or the like, a binder such as a fluororesin or the like, a conductive material, and the like is used. For example, in JP-A-2004-247292, there has been disclosed a technique in which the cycle characteristics can be improved by adding a precipitation inhibitor containing a copolymer of α-olefin and α,β-unsaturated carboxylic acid into the positive electrode mixture.